Science Puzzle
Peto’s Paradox
Cancer begins when a single cell makes a copying error during division and starts to grow uncontrollably. A blue whale has about one thousand times more cells than a human, and lives for decades. Every cell division is a chance for a cancerous mutation.
By simple probability, whales should have an enormous cancer rate compared to mice. Yet large, long-lived animals do not have higher cancer rates than small, short-lived ones. Why not?
The Answer
Large animals have evolved extra layers of cancer suppression. This is Peto's Paradox, named after epidemiologist Richard Peto, who noted in the 1970s that cancer rates do not scale with body size the way simple probability would predict.
The resolution is evolutionary. Any lineage of large animals that did not evolve robust tumour-suppressor mechanisms would have died of cancer before reaching reproductive age, and would have left no descendants. So large body size and long lifespan have been repeatedly accompanied by the evolution of stronger cancer defences.
In elephants, researchers found extra copies of the TP53 tumour-suppressor gene, one of the most important cancer-prevention genes in mammals. Humans have two copies; elephants have about twenty. In whales, multiple additional suppressor pathways have been identified.
Peto's Paradox has practical implications for human medicine: understanding how large animals suppress cancer could reveal new strategies for cancer treatment in humans.
The principle: Peto's Paradox. Large, long-lived animals do not have proportionally higher cancer rates because they evolved stronger tumour-suppressor mechanisms. Without these, they could not have survived to reproduce.